This invention relates to automatic correction systems for color television receivers or CRT monitors and the like and more particularly, to a system for the automatic correction of convergence and gray scale.
Color television receivers of the type employing a cathode ray tube (CRT) having three cathode ray beams and a screen with a mosaic of phosphor dots or stripes of recurring groups of three colors must be adjusted to maintain the convergence and relative beam currents of the three beams over the visible surface of the screen. An analogous adjustment must be made for projection television receivers employing three projection CRTs. These adjustments are initially made at the factory, but with age, temperature and other environmental conditions, it is necessary to readjust convergence and relative beam currents in order to maintain the quality of the reproduced picture. Ordinarily this is accomplished by a skilled technician with test instruments, and the procedure may require the inconvenience of removal of the receiver from the owner's home so that the adjustment can be made at a repair shop.
There are known in the prior art circuits for the automatic control of electron beam intensity to provide for proper color display or color balance. Examples are described in U.S. Pat. No. 2,954,424 issued to Richard Thiele and U.S. Pat. No. 3,479,448 issued to Paul Kollsman.
U.S. Pat. No. 2,954,424 issued to Richard Thiele describes a color television receiver of the type employing a single gun CRT. The screen of the tube is provided with a series of striated vertical color filters in a repetitive sequence such as red, green and blue. Externally of the tube and directed towards its screen is a plurality of photo cells, each sensitive to one color. The photo cells are placed in front of the screen in such a position that the light emitted by the screen can be picked up by the photo cells without interfering with the viewing of the screen. The photo cells are connected together and the output taken through a feedback loop to modulate the grid of the CRT. Also, the photo cells are arranged to be sequentially blanked off so that they cannot convey a signal back through the feedback loop during their blanking time. In operation, if for example the "red" modulated beam sweeps over the screen first, the red light transmitted by the red screen filter elements will irradiate all the photo cells, but the green and blue-sensitive photo cells will not be energized since they are not sensitive to red light. The red-sensitive photo cell is blanked so that no current is fed back to the control grid while the beam is on a red element. However, the scanning beam in its travel will also cause blue and green filter elements to transmit blue and green light, respectively, to the photo cells although the signal bears only "red" modulation. As a result, current will be fed back to the grid of the CRT to suppress the beam while it is passing over the blue and green elements. A similar analysis can be made for the cases of the "blue" and "green" modulated beams. Thus, the Thiele color television receiver merely suppresses the display of those colors which do not correspond to the modulating color signal of the beam. No correction is made for convergence since the Thiele color television receiver employs a single gun CRT.
U.S. Pat. No. 3,479,448 issued to Paul Kollsman describes a system for automatically maintaining the color balance of a scene reproduced by a color television receiver of the type employing a three-gun CRT with a tri-dot phosphor pattern on its screen. In the Kollsman system, a white light spot containing the three primary colors is transmitted from the television station with the program material. This white light spot is reproduced on the screen of the television receiver at a position that is invisible to the viewer. Color imbalance is determined by comparing the ratio of selected reproduced color components of the white spot with a preset reference. Any detected imbalance is automatically corrected by applying appropriate control signals to the screen grids of the CRT. Although the Kollsman system employs a three-gun CRT with a tri-dot phosphor pattern, no mention is made of convergence control. But for the automatic color balance system of Kollsman to work properly, convergence of the three beams is a prerequisite. Moreover, the Kollsman system requires the transmission of a white spot by a cooperating television station.
As will be appreciated by those skilled in the art, correction of convergence is a more difficult problem than correction of relative color intensity, although proper gray scale correction requires the convergence to be in adjustment. The test instruments used to measure convergence often resort to the use of an appliance that is placed over the CRT screen to facilitate detection of the landing point of the cathode ray beam. Such an appliance obscures the screen, and therefore these instruments are not intended to be used simultaneously with the viewing of program material. Examples of such instruments are U.S. Pat. No. 4,001,877 issued to Theodore Frederick Simpson and U.S. Pat. No. 4,035,834 issued to Anthony M. Drury.
U.S. Pat. No. 4,001,877 issued to Theodore Frederick Simpson describes a test instrument that employs a photosensitive array comprising a plurality of individual photo cells, this array being placed over the CRT screen. Further, a special post-deflection coil is required to introduce magnetic fields in the region just forward of the deflection yoke to displace the scanned beams in a controlled pattern from their normal landing points on the screen. The displaced beam causes the emission of an error color, the intensity of which is measured by those photo cells which are sensitive to the error color emitted. The intensity of a reference color emitted by the phosphor deposits stimulated by the undisplaced beam is then measured, and the ratio of the error color to the reference is calculated for each measurement location on the screen. The largest ratio is displayed as an indication of the color purity tolerance of the CRT. The Simpson test instrument is used primarily as a quality control device in the manufacture of color CRTs.
U.S. Pat. No. 4,035,834 issued to Anthony M. Drury describes a beam landing indicator for a color CRT which also employs a holder for positioning a plurality of photo cells over the screen of the CRT. While the Drury instrument does not require a special deflection coil, it does employ a special deflection generator in order to produce a clockwise rotation of the beam landing shift of the beam. This rotation is stepped in increments which occur once each vertical field of the television raster. Light variations sensed by the photo cells are combined with a reference signal to control the dot location on an oscilloscope display of the vector beam landing error. The technician can then make purity adjustments and yoke adjustments of the CRT by observing the oscilloscope display.
Automating the adjustment of color television receivers is also known. An example is described in U.S. Pat. No. 3,962,722 issued to Walter S. Ciciora. More specifically, the Ciciora patent describes a color television setup apparatus for use in the factory. Once again, a holder positions a plurality of photo cells over the CRT screen in such a manner as to obscure the view of the screen. Patterns indicative of the characteristics of contrast, brightness, color and tint are displayed on the CRT. The photo cells develop corresponding electrical signals which are supplied to circuitry that energizes a plurality of bi-directional motors that are engageable with the receiver contrast, tint, brightness and color level adjustment elements.
While the systems described by Simpson, Drury and Ciciora are useful in a factory or shop environment, what is needed is an automatic means for adjustment of convergence and gray scale which is part of the television receiver or CRT monitor. In this way, the receiver or monitor would be continuously maintained in proper adjustment for optimum viewing.